精基准的选择原则 1, the selection principle of fine benchmarks 选择精基准一般应遵循以下几项原则： Generally speaking, the following principles should be followed in selecting fine benchmark. （1）基准重合原则：应尽可能选择被加工表面的设计基准作为精基准，这样可以避免由于基准不重合引起的定位误差。 (1) the principle of reference coincidence: the design datum of the machined surface should be chosen as the fine reference as much as possible so that the positioning error caused by the non coincidence of the datum is avoided. （2）统一基准原则：应尽可能选择用同一组精基准加工工件上尽可能多的表面，以保证各加工表面之间的相对位置精度。 (2) the principle of unified reference: it is necessary to choose as many surfaces as possible with the same group of fine datum to ensure the relative position accuracy between the machined surfaces. （3）互为基准原则：当工件上两个加工表面之间的位置精度要求比较高时，可以采用两个加工表面互为基准反复加工的方法。 (3) the principle of mutual reference: when the position precision between two machined surfaces on the workpiece is higher, the two machining surfaces can be processed repeatedly. （4）自为基准原则：一些表面的精加工工序，要求加工余量小而均匀，常以加工表面自身作为精基准。 (4) self - based principle: some surface finishing processes require small and uniform machining allowance, often using the machined surface itself as a fine benchmark. 上述四项选择精基准的原则，有时不可能同时满足，应根据实际条件决定取舍。 The above four principles of selecting fine benchmarks are sometimes impossible to meet at the same time, and decisions should be made according to actual conditions. 2、粗基准的选择原则 2, the principle of selecting rough benchmarks （1）工件加工的第一道工序要用粗基准，粗基准选择得正确与否，不但与第一道工序的加工有关，而且还将对工件加工的全过程产生重大影响。 (1) the first process of workpiece processing should be used with coarse datum, and the selection of rough datum is correct or not. It is not only related to the processing of the first process, but also has a great influence on the whole process of the workpiece. （2）合理分配加工余量的原则：从保证重要表面加工余量均匀考虑，应选择重要表面作粗基准。 (2) the principle of reasonable allocation of machining allowance: from the uniform consideration of ensuring important surface machining allowance, the important surface should be selected as the rough datum. （3）便于装夹的原则：为使工件定位稳定，夹紧可靠，要求所选用的粗基准尽可能平整、光洁，不允许有锻造飞边、铸造浇冒口切痕或其它缺陷，并有足够的支承面积。 (3) the principle of easy to clamp: in order to make the workpiece stable, clamped and reliable, the coarse reference is required to be as smooth and clean as possible. It is not allowed to have forged flown, cast in situ riser cut marks or other defects, and has sufficient supporting area. （4）粗基准一般不得重复使用的原则。 (4) rough datum should not be repeated in principle. 上述4项选择粗基准的原则，有时不能同时兼顾，只能根据主次决择。 The above 4 principles of selecting rough benchmarks can not be considered at the same time